This invention relates to a technique for improving the surface characteristics of water-cooled rolls to be used for cooling steel sheets in heat-treating furnaces.
Continuous annealing furnaces for steel sheets are provided with a quenching zone to help produce well-aged cold-rolled sheets or the like. One method of cooling in that zone uses water-cooled rolls.
FIG. 1 illustrates the concept of the roll cooling. An array of internally water-cooled metal rolls 1 cools a steel sheet 2 as the latter passes in direct contact with the rolls, under control such that the work is cooled down to a given finish temperature at a controlled rate.
The water-cooled rolls hitherto used have been metal rolls. The metal ones have not been fully satisfactory. For one thing, they have questionable durability to cope with the heat cycles involving contact with high-temperature steel sheets and internal water cooling and, for the other, they are not quite resistant to the surface wear due to friction with the steel sheets usually conveyed under tension ranging from about 0.5 to about 3 kg/cm.sup.2.
In view of this, it has already been proposed to reinforce the water-cooled rolls with metal carbide coatings (Utility Model Application Publication No. 19317/1988) or metal oxide coatings (Patent Application Public Disclosure No. 136634/1986).
However, metal carbide coatings have high thermal conductivity values, and the non-uniformity of surface roughness has an adverse affect on the local rate of heat transfer. This can result in an uneven rate of cooling of the steel sheets.
It was to eliminate this disadvantage that spray coating with metal oxides was proposed. The metal oxides are low enough in thermal conductivity to prevent the non-uniformity of surface roughness from influencing the uniformity of the cooling rate. The metal oxide coatings, however, exhibit such poor peeling resistance under service conditions. In addition they frequently require a double-layer bonding coat of about 200 .mu.m thick. If desirable effects are to be achieved, the sprayed metal oxide coating itself must have a thickness of at least 200 .mu.m.